E. coli is one of the leading causes of diarrhea in children, is a major endemic health threat in the developing world, and is responsible for the yearly death of several hundred thousand people. After binding to the cells that line the intestine, E. coli injects several proteins that result in diarrhea and disease. The structural and functional properties of these proteins will be characterized, initially focusing on the EspG protein, which is postulated to remodel host microtubule structure. The hypothesis that one or more translocated proteins bind to host DNA, to repress the expression of genes involved in host immunity, will also be tested, using epitope-tagging and immunofluorescence microscopy. This important concept remains unexplored in human enteric pathogens. The host response to EPEC infection in cultured polarized epithelial cells will also be characterized, using DNA microarray technology. Changes in host gene expression profiles will be examined, to learn which cellular pathways are most profoundly altered upon infection. Such studies have not been carefully performed with polarized epithelial cells. The characterization of toxic E. coli proteins and the analysis of the intestinal response to these proteins will guide future vaccine and antibiotic designs.